Air brake systems are frequently employed for heavy-duty vehicles such as tractor trailers and the like. In such systems, the brake system is activated by pressurized air transported through tubing. Metal tubing, of course, can be employed. However, generally nylon tubing is used and in many applications, reinforced nylon tubing is required. By industry standards, reinforced nylon tubing must include an inner nylon layer and an outer nylon layer with an intermediate polyester or nylon reinforcing layer. This tube is designed to operate at a maximum pressure of 150 psi over a wide temperature range i.e. -40.degree. C. to 90.degree. C. The test requirements are even more strenuous requiring the tubing to withstand 150 psi pressure at temperatures from -40.degree. up to 110C.
In the past, such polyester reinforced nylon tubing has performed quite well. It meets the minimum requirements with respect to temperature and pressure and generally exceeds these. In certain applications, it may be desirable to further broaden the temperature characteristics of such tubing. Further nylon itself is relatively expensive. High density polyethylene (HDPE) on the other hand has greater temperature range and is significantly less expensive than nylon. Nylon has been utilized for an extended period of time in automotive and trucking applications and therefore it's characteristics with respect to fuel stability and the like are well known. Therefore air brake tubing still requires that the inner and outer layers be nylon.
If one were to simply utilize a layer of high density polyethylene within the reinforced nylon brake tubing, in turn reducing the amount of nylon, the formed product will tend to delaminate if repeatedly subjected to high pressures. In effect, stress cracking occurs where the polyester strand contacts the HDPE layer.
Accordingly, it is an object of the present invention to provide a laminated, reinforced tubing which does not delaminate. Further, it is an object of the present invention to provide such a laminated air brake tubing that has improved low temperature characteristics. Further, it is an object of the present invention to provide such a laminated tubing utilizing layer of a polyolefin preferably high density polyethylene, HDPE.
The present invention is premised on the realization that such a tubing can be formed wherein the polyolefin layer is separated from the polyester layer with a protective layer. Preferably, according to the present invention, such a laminated tubing includes an inner layer of nylon, an intermediate HDPE layer, a braided polyester layer, and an outer nylon layer plus a protective layer which separates the HDPE from the polyester braiding.
In a preferred embodiment of the present invention, the laminated tubing includes two layers of HDPE, one on either side of the polyester braiding with protective layers separating the polyester from the HDPE. Although a variety of protective layers could be employed, the preferred protective layer is a thin layer of nylon, preferably, nylon 11 or 12.